Heteroaggregation corresponds to an attraction between two different types of particles. It can either lead to stable or instable systems, depending on the conditions. In this paper heteroaggregation in binary mixtures of alumina and silica colloids is studied, by coupling an experimental approach and Brownian dynamics simulations. Two main objectives are targeted. The first one is to bridge the gap between coagulation and dispersion, by examining the effect of the relative concentrations of silica and alumina. The second one is to study the effect of the volume fraction of solid, which impacts strongly the aggregation mechanism. The coagulation or the dispersion are evidenced by sedimentation tests, granulometry, and rheological measurements, supported by zeta potential measurements. Heteroaggregates could be observed by transmission electron microscopy. Brownian dynamics simulations give more insight into the very first moments of the process, to predict the adsorption kinetic and the heteroaggregates structure. A very good agreement is obtained between experiments and simulations, which both show adsorption of silica leads to the bridging of alumina particles and thus coagulation at low concentration. This adsorption is rapidly limited by electrostatic interactions, then hindering alumina agglomeration by repulsive interactions. For the higher solid fractions, alumina aggregation is more likely, as two alumina particles can easier encounter before their surface is enough covered by the silica to avoid their aggregation.

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Numerical simulations constitute a precious tool for understanding the role of key parameters influencing the colloidal arrangement in suspensions, which is crucial for many applications. The present paper investigates numerically the role of hydrodynamic interactions on the aggregation processes in colloidal suspensions. Three simulation techniques are used: Brownian dynamics without hydrodynamic interactions, Brownian dynamics including some of the hydrodynamic interactions, using the Yamakawa-Rotne- Prager tensor, and stochastic rotation dynamics coupled with molecular dynamics. A system of monodisperse colloids strongly interacting through a generalized Lennard-Jones potential is studied for a colloid volume fraction ranging from 2.5 to 20%. Interestingly, effects of the hydrodynamic interactions are shown in the details of the aggregation processes. It is observed that the hydrodynamic interactions slow down the aggregation kinetics in the initial nucleation stage, while they speed up the next cluster coalescence stage. It is shown that the latter is due to an enhanced cluster diffusion in the simulations including hydrodynamic interactions. The higher the colloid volume fraction, the more pronounced the effects on the aggregation kinetics. It is also observed that hydrodynamic interactions slow down the reorganization kinetics. It turns out that the Brownian dynamics technique using the Yamakawa-Rotne-Prager tensor tends to overestimate the effects on cluster diffusion and cluster reorganization, even if it can be a method of choice for very dilute suspensions.

We consider tracer diffusion in colloidal suspensions under solid loading conditions, where hydrodynamic interactions play an important role. To this end, we carry out computer simulations based on the hybrid stochastic rotation dynamics-molecular dynamics (SRD-MD) technique. Many details of the simulation method are discussed in detail. In particular, our choices for the SRD-MD parameters and for the different scales are adapted to simulating colloidal suspensions under realistic conditions. Our simulation data are compared with published theoretical, experimental and numerical results and compared to Brownian dynamics simulation data. We demonstrate that our SRD-MD simulations reproduce many features of the hydrodynamics in colloidal fluids under finite loading. In particular, finite-size effects and the diffusive behavior of colloids for a range of volume fractions of the suspension show that hydrodynamic interactions are correctly included within the SRD-MD technique.